The most common form of marking or coloring ceramic materials involves coating the surface of a ceramic article, fired or unfired (green), with a dispersion of ceramic pigments and firing the coated article to obtain permanent coloration. The ceramic material, ceramic pigments, dispersion additives and firing conditions can vary widely. For example, the ceramic materials can include clay-based ceramics, glass ceramics and refractory glassware; the ceramic pigments can include a vast array of alkali metal oxides, transition metal oxides and mixtures thereof and the dispersion additives can include glazing materials and fluxes to form a vitrifiable mixture or additives such that the pigment dispersion is non-vitrifiable (stain).
The most common method for applying the ceramic pigment is dipping, spraying or painting the ceramic article with a dispersion of the pigment within a volatile liquid carrier and evaporating the liquid carrier. These techniques do not allow for fine patterning due to limited control over the coating step and in some cases due to running of the pigment dispersion during firing.
Decals have been used to decorate ceramics with fine patterns. As disclosed by Kluge et al. in U.S. Pat. No. 3,894,167, to prepare these decals, an image or marking is printed on a substrate, such as paper, wherein the printed image contains a ceramic pigment. This printed image is then transferred to a ceramic article and fired. Kluge et al. disclose that the print is preferably overcoated with a lacquer to provide adhesion to the ceramic. To release the printed image from the paper substrate to the ceramic article, the paper is typically wet or a wax is added to the paper. The unremoved substrate and any organic components are burned away when fired. Kluge et al. describe decals wherein a wax is incorporated in the print itself to aid separation from the substrate. Unlike thermal transfer printing, the entire wax layer is transferred to the receiving substrate without differentiation and without heating.
Kluge indicates that the printed patterns can be obtained using liquid inks in lithography, off-set printing and screen-printing. Hayakawa et al. (U.S. Pat. No. 5,421,877) disclose a ceramic color composition applied to glass surfaces by screen printing. These printing methods require significant preparation and effort to perform.
In screen printing, a stencil representing the non-printing areas is prepared and applied to a silk or nylon fine-mesh screen. Ink is applied to the patterned screen by scrapping with a rubber squeegee and the ink is then transferred to a receiving substrate. In lithographic printing a pattern is formed on an ink carrier. Liquid ink is distributed to water receptive areas of the carrier such as a drum carrier to the exclusion of water repellent areas (grease receptive). The patterned ink on the carrier is then transferred to a receiving substrate. Off-set printing works on the same principle as lithographic printing requiring the preparation of a pattern on an ink carrier. However, the ink is transferred to an intermediate carrier before being transferred to the desired substrate. These printing methods require significant preparation prior to printing in that a patterned screen or ink carrier must be prepared. In contrast, thermal transfer printing can be characterized as an "on demand" printing method in that it requires little preparation for a new image.
In thermal transfer printing, images are formed on a receiving substrate by heating extremely precise areas of a print ribbon with thin film resistors. This heating of the localized area causes transfer of pigment from the ribbon to the receiving substrate. Thermal transfer printing has displaced many forms of printing such as lithography due to its speed and flexibility in providing printed images on demand. Thermal transfer printing has also displaced impact printing in many applications due to advances such as the relatively low noise levels which are attained during the printing operation.
The thermal transfer process provides great flexibility in generating images and allows for broad variations in style, size and color of the printed image. Representative documentation in the area of thermal printing includes the following patents:
U.S. Pat. No. 3,663,278, issued to J. H. Blose et al. on May 16, 1972, discloses a thermal transfer medium comprising a base with a coating comprising of cellulosic polymer, thermoplastic aminotriazine-sulfonamide-aldehyde resin, plasticizer and a "sensible" material such as a dye or pigment. PA1 U.S. Pat. No. 4,315,643, issued to Y. Tokunaga et al. on Feb. 16, 1982, discloses a thermal transfer element comprising a foundation, a color developing layer and a hot melt ink layer. The ink layer includes heat conductive material and a solid wax as a binder material. PA1 U.S. Pat. No. 4,403,224, issued to R. C. Winowski on Sep. 6, 1983, discloses a surface recording layer comprising a resin binder, a pigment dispersed in the binder, and a smudge inhibitor incorporated into and dispersed throughout the surface recording layer, or applied to the surface recording layer as a separate coating. PA1 U.S. Pat. No. 4,463,034, issued to Y. Tokunaga et al. on Jul. 31, 1984, discloses a heat-sensitive magnetic transfer element having a hot melt or a solvent coating. PA1 U.S. Pat. No. 4,687,701, issued to K. Knirsch et al. on Aug. 18, 1987, discloses a heat sensitive inked element using a blend of thermoplastic resins and waxes. PA1 U.S. Pat. No. 4,707,395, issued to S. Ueyama et al., on Nov. 17, 1987, discloses a substrate, a heat-sensitive releasing layer, a coloring agent layer, and a heat-sensitive cohesive layer. PA1 U.S. Pat. No. 4,777,079, issued to M. Nagamoto et al. on Oct. 11, 1988, discloses an image transfer type thermosensitive recording medium using thermosoftening resins and a coloring agent. PA1 U.S. Pat. Nos. 5,128,308 and 5,248,652, issued to Talvalkar, each disclose a thermal transfer ribbon having a reactive dye which generates color when exposed to heat from a thermal transfer printer. PA1 U.S. Pat. No. 5,240,781, issued to Obatta et al., discloses an ink ribbon for thermal transfer printers having a thermal transfer layer comprising a wax-like substance as a main component and a thermoplastic adhesive layer having a film forming property. PA1 said formulation comprising wax, binder resin, optionally solvent and a sensible material comprising ceramic pigment in an amount sufficient to form thermal transfer layers that will provide printed images which can permanently mark a ceramic material when said printed image is applied to the surface of said ceramic material and fired. PA1 said thermal transfer medium comprising a flexible substrate with a thermal transfer layer positioned thereon, said thermal transfer layer comprising wax, binder resin, sometimes residual solvent, and a sensible material comprising a ceramic pigment in an amount sufficient to provide images that can permanently mark ceramic materials when applied to the surface of a ceramic material and fired.
It is desirable to take advantage of the quality, versatility and economy of thermal transfer printing in permanently marking ceramic articles.